The Institute of Electrical and Electronics Engineers (IEEE) has adopted a set of standards for wireless local area networks (WLANs), known as 802.11, as well a set of standards for wireless metropolitan area networks (WMANs), known as 802.16. Wireless products satisfying the 802.11 and 802.16 standards are currently on the market, for example. The term, WiFi, is used herein to describe equipment satisfying the 802.11 standard. The term, WiMAX, short for worldwide interoperability for microwave access, is used herein to describe equipment satisfying the 802.16 standard.
In wireless transmitter applications, particularly WiMAX, it is desired to achieve high output power combined with high-efficiency to save the power of the portable device. An out-phasing transmitter architecture is based on decomposing the input signal into two constant amplitude signals driving two high-efficiency switching amplifiers. However, the dynamic range of this classical out-phasing architecture is limited by the mapping of the amplitude signal into the phase difference between the two signals driving the amplifiers. This mapping limits the dynamic range to 35 dB for 0° to 89° of phase difference (0° corresponds to maximum power condition and 89° corresponds to −35 dB from maximum power).
FIG. 1 shows a classical out-phasing power amplifier 10 composed of two switching amplifiers 12A, 12B (collectively, switching amplifiers 12), with two quarter-wave transmission lines 14A, 14B, for power combining, and a common load 20, according to the prior art. The equations, which describe how to extract two constant amplitude signals, S1(t) and S2(t), to drive the two switching amplifiers 12, are shown at the left side of FIG. 1.
The dynamic range of this classical system is limited by the matching accuracy of the two transmission paths after applying any required calibration. For example, a dynamic range of 35 dB is achieved by generating out-phasing signals between 0° and 89° (note that 0° is the condition of maximum power or in-phase condition). If the dynamic range is to be extended further, any smaller amplitude signal has to be mapped within 89° to 90°, which is extremely difficult to achieve even with calibration, due to the non-idealities of the calibration system itself. Moreover, the matching accuracy is a function of process variations and temperature distribution within the die. All of these factors limit the dynamic range of the output signal to roughly 35 dB, and cause the transmitter to be very sensitive to phase (and gain) mismatches between the two paths. FIG. 2 shows a mapping 30 between the input power and the out-phasing angle (10 dB is the maximum input power condition), according to the prior art.
Thus, there is a continuing need for a WiMAX transmitter architecture that overcomes the shortcomings of the prior art.